In recent years, lithium secondary batteries have been widely used as driving power supplies for small electronic devices and the like. Such lithium secondary batteries are mainly constituted of a positive electrode, a nonaqueous electrolyte solution and a negative electrode. In particular, lithium secondary batteries constituted of a lithium compound oxide such as LiCoO2 as the positive electrode and a carbon material or a lithium metal as the negative electrode are used. The nonaqueous electrolyte solution for use in such lithium secondary batteries includes a carbonate such as ethylene carbonate (EC) or propylene carbonate (PC).
There is, however, a need for secondary batteries having better characteristics such as battery cycling property and electrical capacity.
A lithium secondary battery using, for example, LiCoO2, LiMn2O4 or LiNiO2 as a positive electrode brings about a reduction of the battery performance, when a part of the solvent of the nonaqueous electrolyte solution locally undergoes an oxidative decomposition during the charging, because the decomposition products inhibit the desired electrochemical reaction of the battery. Such a reduction is considered to be attributed to an electrochemical oxidation of the solvent at the interface between the positive electrode material and the nonaqueous electrolyte solution.
Also, a lithium secondary battery using, for example, a highly crystallized carbon material, such as natural graphite or artificial graphite, as a negative electrode brings about a reduction of the battery performance, when the solvent of the nonaqueous electrolyte solution undergoes a reductive decomposition on the surface of the negative electrode during the charging. Even in the case of ethylene carbonate (EC) which is widely generally used as a solvent for the nonaqueous electrolyte solution, a part thereof undergoes a reductive decomposition during repeated charging and discharging. Propylene carbonate (PC) having a low melting point and a high dielectric constant is preferable for use as a nonaqueous solvent because of its high electric conductivity even at low temperatures. However, PC has not been used for lithium secondary batteries because the decomposition thereof is significant.
For the purpose of improving the battery characteristics of such lithium secondary batteries, a variety of proposals have been made.
For example, Patent document 1 discloses an electrolyte solution containing the first solvent such as propylene carbonate, the second solvent such as dimethyl carbonate or diethyl carbonate, and vinylene carbonate in an amount of 0.01 to 10% by weight based on the solvent mixture. Patent document 2 discloses an electrolyte solution containing 0.1 to 5% by weight of vinylene carbonate and an asymmetrical linear carbonate.
Patent document 3 discloses a nonaqueous electrolyte solution containing a mixed solvent of ethylene carbonate and dimethyl carbonate to which a diester of a dicarboxylic acid having an unsaturated group, such as diethyl acetylenedicarboxylate, is added.
Patent document 4 discloses a nonaqueous electrolyte solution containing a diester of acetylenedicarboxylic acid having a specific structure. This compound differs from a carboxylate compound used in the present invention. Further, no consideration is made on combination with a nonaqueous solvent.
Patent document 5 discloses an electrolyte solution containing a cyclic carbonate and a linear carbonate, as main ingredients, and 0.1 to 4% by weight of 1,3-propane sultone.
The techniques disclosed in Patent documents 1 to 5, however, cannot satisfy the battery characteristics required in recent years. There is a demand for an electrolyte solution having more excellent cycling characteristics for providing lithium secondary batteries with high capacity.
Patent document 1: Japanese Unexamined Patent Publication H08-45545
Patent document 2: Japanese Unexamined Patent Publication H11-185806
Patent document 3: Japanese Unexamined Patent Publication 2001-256995
Patent document 4: Japanese Unexamined Patent Publication 2003-59532
Patent document 5: Japanese Unexamined Patent Publication 2000-3724